Below you find a complete list and descriptions of the new functions in the latest FRITZ!OS, along with a list of further improvements in this version.
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What's new in FRITZ!OS (from 6.50)

DSL:
NEW-With active VDSL vectoring, impulse noise protection (G.INP) can be used at the same time as seamless rate adaptation (SRA)

Internet:
NEW: Parental controls with tickets to extend online time as needed
NEW: Display of remaining online time and restrictions to Internet use with fritz.box/surf.lua (parental controls)
NEW: Display of a longer Internet disruption (red INFO LED and on Overview page of the user interface)
NEW: In the DS-Lite connection mode, the Port Control Protocol can be used to configure incoming IPv4 port sharing in the AFTR gateway of the Internet service provider. Must be supported by the Internet service provider.
NEW: Use own local DNS server (IPv4 and IPv6)
NEW: Automated port sharing in the home network set up via the Port Control Protocol (PCP)
NEW: New filter for TCP port 25 to block unsecured e-mail dispatch (can be enabled under "Internet / Filters / Lists")

Wireless:
NEW: Displays loading of radio channel currently in use (temporal progress)
NEW: Option for granting Internet access via the guest access only after consent to terms of use (captive portal)
NEW: Wireless standard in the 2.4-GHz band set to "802.11n+g" (presetting changed)
NEW: Failed login attempts by wireless devices recorded in a separate list on the "Wireless / Radio Network" page.

Storage (NAS):
NEW-USB storage media keep the drive name set on the computer (not active until factory settings configured)

Home network:
NEW: "Home network overview" of all connected devices
NEW: Central update function for all FRITZ! products in the home network
NEW: Deletion of inactive devices without special settings from the overview of network connections
NEW: Secure access to the user interface from connected FRITZ! devices via the FRITZ!Box password
NEW: Display of VPN users in the network connections overview

System:
NEW: User interface in new responsive design for optimized display on smartphones and tablets
NEW: Completely new graphic design of the user interface
NEW: "Change notice" push service notifies about changes to selected settings and events
NEW: Push service sends mail with status after successful update (effective after next update)
NEW: Display of important notifications for secure, reliable operation with red "Info"' LED and on the start page

Security:
NEW: For increased security when accessing the user interface, now a FRITZ!Box password must be assigned during the first configuration
NEW: For increased security, a password must be assigned to backup files
NEW: Firewall "stealth mode" setting available under Internet > Filters > Lists > Global Filter Settings

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New features for the FRITZ!Box: New design for FRITZ!OS 6.50, with better overview of the home network and over 120 innovations

The latest update offers a redesigned user interface at http://fritz.box, optimized for smartphones, tablets and notebooks. In the browser it adjusts flexibly to the different display sizes. The user interface also received a fresh new look. The menu items and their arrangement remain familiar to users.

The extensive home network overview of all connected devices is also new. These include all wireless, LAN, powerline and USB devices. At a single glance users can see whether their connected FRITZ! products are equipped with the latest version of FRITZ!OS. You can start updates directly from here.

With the new FRITZ!OS you can also use helpful analyses of your wireless LAN environment. Under "Wireless / Radio Channel" the loading of the radio channel currently in use is shown over time for the 2.4-GHz and the 5-GHz frequency band. The lower the loading of the radio channel, the better the potential data throughput there.

Now there is an option for granting guest access to the wireless LAN or LAN only after consent to the FRITZ!Box owner's terms of use. The guest is taken first to a captive portal to signal consent. Without confirming this page, you cannot connect to the Internet.

News for parental controls: Tickets can be issued for each device to extend surfing time by 45 minutes. When online time has been exhausted, tickets can be redeemed on the Information page on blocked Internet use in the browser.
By going to http://fritz.box/surf.lua with any network device, you can view the device's settings for Internet use and the amount of online time already used on the current day.

Since AVM also optimizes the security of the entire FRITZ! family with every new FRITZ!OS, it is advisable for every user to perform the update. The firewall can be operated in stealth mode to prevent ping queries. TLS security has been improved and port 25 received a new filter to block the unsecured e-mail dispatch.

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Improvements and Bug Fixes in FRITZ!OS 6.52

Internet:
Improved: Wireless LAN stability for using Entertain

Home network:
Resolved: Fixed assignment of the same IPv4 address lost on restart

System:
Improved: Stability, eliminated possibility of continual reboots in special application scenarios
Changed: Update with a FRITZ!OS file is permitted only with the latest version of FRITZ!OS

Improvements and Bug Fixes in FRITZ!OS 6.50

DSL:
Improvement-Notice on login page when no DSL connection available
Resolved-impulse noise protection (G.INP) was not negotiated during first connection after restart
Resolved-problem connecting to Broadcom 147.159 (VDSL)
Resolved-problem connecting to Infineon 7.23.4 (ADSL2+)

Wireless:
Improved: Display of wireless LAN channels occupied by adjacent networks
Improved: Maximum possible data rate shown on the detail pages of wireless LAN devices in "Wireless / Radio Network"
Improved: Password evaluation for entry of wireless network key
Improved: Reworked messages on wireless LAN under "System / Events"
Improved: Wireless auto-channel function now also occupies higher channels in the 5-GHz band
Improved: Wireless guest access now encrypted with WPA2 instead of WPA/WPA2 (mixed mode)
Resolved: In "Repeater" operating mode, changes to "Radio Channel Settings" led to permanent loss of connection to access point
Resolved: Transmission power for enabled guest access can be controlled again
Resolved: Wireless guest access can be enabled in the "Share existing Internet connection in the network (IP client)" mode again

Storage (NAS):
Improved: Warning message on attempt to delete the "FRITZ!" folder
Improved: Media server supports thumbnails for contents of Synology NAS devices
Resolved: After releasing a directory for sharing with a user, NAS could no longer be reached by other users

Home network:
Improved: Display of type and path of a device's connection to the FRITZ!Box now included in the details of the "Network Overview" page
Improved: "Network Overview" page with optimized listing of powerline devices with their own user interface
Improved: Reworked local IPv6 settings
Improved: All AVM FRITZ!Powerline products in the powerline network can now be updated from the FRITZ!Box user interface

System:
Improved: Upon conclusion, configuration wizard checks whether a new FRITZ!OS is available and offers an update
Improved: Reworked content and display of push service mails
Improved: Link to the FRITZ!Box user interface in the footer of the push service mail can now be accessed externally as well (requires MyFRITZ! or Dynamic DNS)
Improved: Option to deactivate push service entirely
Improved: Settings backup now also includes parental controls, wireless guest access and basic smart home settings
Improved: Sorting function in tables in the user interface
Improved: Dispatch of "FRITZ!Box Info" push mail as needed
Improved: "FRITZ!Box Info" and "Wireless Guest Access" push mail sent before restart
Improved: Time-saving operation via optimized navigation in the user interface
Improved: Display of new functions via HTTPS before a FRITZ!OS update
Improved: New event for saving settings

Security:
Improved: Password evaluation during first configuration and configuration of MyFRITZ!
Improved: Secure TLS mandatory, support for SSLv3 removed

Diagnostics:
Improved: Function diagnosis of function takes own FRITZ!WLAN Repeaters into consideration when counting radio networks of that name in the vicinity
Improved: Function diagnosis now includes detection of old wireless LAN devices (802.11b)
Improved: Send diagnostics results for "Function" and "Security" by mail

We recommend every update for reasons of security as well. Further improvements to increase security with this version of FRITZ!OS are presented at
http://en.avm.de/guide/security in the area "Updates increase security".

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New in FRITZ!OS 6.30

This version of FRITZ!OS includes modifications to better support the latest services offered by Internet providers and improved interoperability on the DSL line.

Vodafone customers can enjoy an extra improvement with the new FRITZ!OS. It is now even easier to reconfigure the Vodafone account data entered during the first installation of the FRITZ!Box for a new DSL/VDSL line.

Additional improvements to the DSL software provide for improved stability and faster transmission over various connections. The DSL diagnostics data have also been expanded for error analysis, so that future optimizations can be analyzed in even greater detail.
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Improvements and Bug Fixes in FRITZ!OS 6.30

Further improvements to increase security with this version of FRITZ!OS are presented at en.avm.de/guide/security in the "Updates increase security" area.
Access to the FRITZ!Box via Telnet is no longer supported
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New with FRITZ!OS 6.20:

DSL: see how long no signal was received on the DSL information page
DSL: VDSL Interop improvements
DSL: improved stability for G.Vector
DSL: improved DSL Diagnosis

Home net: display of MIMO standard in the detailed information about powerline devices
Home net: use of fixed IP addresses via DHCP server improved
Home net: change parental controls profile in the detailed settings for every network device
Home net: BPjM (Bundesprüfstelle für Jugendgefährdende Medien) filter is now only active if country is set to 'germany'

Internet: VPN connection to Android devices no longer disconnected after one hour
Internet: online counter now able to display higher bandwidths
Internet: when activating MyFRITZ! a random port for https will be selected
Internet: fixes a possibility to circumvent parental controls via guest access
Internet: added Diffie Hellmann Group 5, 14 and 15 for VPN
Internet: TR-069 connection request port is now closed once TR-069 has been deactivated
Internet: improved accessibility in IP client mode
Internet: added possiblity to use public IPv4 subnet
Internet: support for more 3G dongles with USB tethering
Internet: BPjM filter only active when country is set to Germany

Media server: support for Google "all inclusive" music added
Media server: support for Google Play Music Playlists

Security: enhanced security area in wireless LAN settings
Security: assistance in choosing safe passwords
Security: push mails are now sent using SSL encryption by default
Security: import and export the SSL certificate used by your FRITZ!Box

System: notification for important FRITZ!OS updates
System: crashes when using the new push services fixed
System: enables password to be set for the saved settings before an update
System: new FRITZ!OS Updates can be installed automatically (default setting)*
System: several improvements for the update process
System: improved scheduled switching for tablets and smartphones.

System
NEW: improved and expanded push service
NEW: keylock for WLAN key on the FRITZ!Box
NEW: display of information for wireless LAN guest access and connected devices

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New with FRITZ!OS 5.52:
-Security: removes possibility for unauthorized access to FRITZ!Box.

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To carry out the firmware update:

1) Open your Internet browser and enter the address "fritz.box" or "192.168.178.1"
2) Select the menu "System" and then the sub-menu "Firmware Update".
3) Use the "Browse ..." button to enter the appropriate path to the update file.
4) Click the "Update" button.

To recover the storage space on your hard drive, you can delete the update file after
you have carried out the update.

This firmware package consists of files which are distributed under different
license terms, in particular under AVM proprietary license or under any Open
Source License (namely GNU General Public License, GNU Lesser General Public
License or FreeBSD License). For further details see file "license.txt"
(ftp://ftp.avm.de/fritz.box/license.txt). The source
code of those files distributed as Open Source are available on written request
to fritzbox_info@avm.de.

Under all AVM intellectual property rights, AVM grants the non-exclusive right
to personally use this AVM firmware package which is delivered in object code
format only. Licensee shall olny be entitled to make a copy exclusively reserved
for personal backup purposes (backup copy). AVM reserves all intellectual
property rights except as expressly granted herein. Without the prior written
approval of AVM and except to the extent as may be expressly authorised under
mandatory law, this AVM firmware package in particular

- shall not be copied, distributed or otherwise made publicly available
- shall not be modified, disassembled, reverse engineered, decompiled or
otherwise "be opened" in whole or in part, and insofar shall not be copied,
distributed or otherwise made publicly available.